This FIRCA proposal describes a systematic and detailed series of in vitro pharmacological studies designed to evaluate the complex bidirectional (facilitatory and inhibitory) modulation of the stimulation-evoked release of substance P (SP) from sensory structures. We have previously demonstrated that opioids, such as morphine, produce a concentration-dependent activation of mu-1, mu, delta and kappa-opioid receptors. In the proposed studies, we will look at the release of SP evoked by high K+ concentrations, electrical stimulation and capsaicin from rat trigeminal nucleus slices. This model will enable us to determine which releasable source(s) of SP in the trigeminal nucleus is(are) modulated by opioid-receptor agonists. Then, we will elucidate whether morphine's bidirectional modulation of SP release is a direct effect mediated by opioid receptors located on SP-containing terminals or if it is an indirect effect mediated by opioid-receptors located on neighboring neurons. Finally, we will study the opioid modulation of SP release evoked by electrical stimulation that more closely approximates physiological conditions. The information gained from the proposed collaboration should contribute to our current knowledge on mechanisms underlying nociception, analgesia and hyperalgesia at the spinal level as well as to design and implement novel, well controlled clinical studies that will assess the functional significance of the experimental outcomes provided by the collaborating institution.